What is Slate?

The common name for a bluish fossil stone, very soft when dug out of the quarry, and easily cut or split into thin plates,--a property which renders it invaluable for a variety of purposes.

_Invaluable_, extremely valuable.

For what is it used?

Slate has superseded the use of lead for covering roofs, even of the largest buildings; being lighter and more durable, it is preferable to tile: it is also employed for slabs to form cisterns, shelves for dairies, and other purposes, on account of its strength, coolness, and the ease with which it can be cleaned; the latter quality renders it also of great value in the business of education, as a cheap subst.i.tute for paper. The ancients were unacquainted with the use of slate.

What other kinds of stone are used in building?

Limestone, or the calcareous rocks of the geologist: of these there are many varieties. Those which are easily cut and polished are termed marbles, and are used in sculpture and in ornamental architecture. The coa.r.s.er marbles are used for the common purposes of building.

_Calcareous_, partaking of the nature of calx or lime,--a term employed to describe chalk, marble, and all other combinations of lime with carbonic acid.

_Geologist_, one who studies the science of Geology.

Of what do Calcareous Earths or Stones consist?

Calcareous earths, stones, or rocks consist of lime, or pure calcareous earth, carbonic acid, and water.

What is Quick-Lime?

Limestone deprived of its carbonic acid and water by being subjected to an intense heat in a kiln.

How are these Stones wrought?

To whatever purpose the stones are to be applied, the larger blocks obtained from the quarry must be cut into smaller and more manageable pieces by sawing: the saw used is a long blade of steel, without teeth, fixed in a heavy wooden frame. These huge saws are worked by one or two men who sit in boxes to shelter them from the weather; water is caused to drip constantly into the cut, to facilitate the motion of the saw, and keep it cool, so as to prevent it from losing its temper.

_Huge_, very large.

_Temper_, hardness; in speaking of metals it signifies the state to which they are reduced, especially with regard to their hardness.

What is Steel?

Iron combined with a small portion of carbon; its chemical name is _Carburet of Iron_. It is not so malleable as iron in its ordinary state; but is much harder, more elastic, and susceptible of a higher polish. Of this material are manufactured knives, swords, and all kinds of cutting instruments and edge tools, used for domestic purposes and in the arts, from the ponderous pit-saw to the finest lancet. Good steel is much more ductile than iron; and a finer wire may be drawn from it than from any other metal. The excellence of edge-tools depends upon their temper.

_Ponderous_, heavy.

You say that a Geologist is one who studies Geology: what is meant by this term?

A science which enables us to read, in the simple language of nature, the changes which have taken place on the surface of the earth, in its structure and mineral const.i.tution. It describes the different materials and the strata of which the crust of the earth is composed, and investigates the causes of its physical features.

_Simple_, easily read.

What are Strata?

Layers of rocks and other substances of which the whole earth seems to be composed. These rocks are found lying one above another in regular order; beneath them are the _unstratified_ rocks, which seem to form the basis or foundations upon which the others have been deposited.

The various layers seem to have been formed during progressive stages of vegetable and animal organization. These rocks and strata are divided into five cla.s.ses or formations.

_Progressive_, moving forwards.

_Organization_, formation or structure of bodies.

Name them.

The Primitive, or lower formations, supposed to have been formed in the chaotic state of the earth, because they have no trace of organized beings or petrifactions; they are chiefly composed of silicious and argillaceous earths, as granite, slate, &c.--Transition rocks, supposed to have been formed during the transition of the earth into a habitable state; they differ from the primitive, in containing the remains of marine animals:--the Secondary rocks, containing the remains of animals and vegetables, and consequently formed after their creation;--the Tertiary formation, composed of layers of clay, sand, gravel, and marl, and containing peculiar organic remains;--and the Alluvial formation, const.i.tuted of parts of previous rocks separated by water, &c., and deposited in beds.

_Petrifaction_, an animal or vegetable substance turned to stone.

_Silicious_, consisting of flint.

_Transition_, change from one state to another.

_Argillaceous_, clayey, consisting of clay.

_Chaotic_, resembling chaos, confused.

_Chaos_, confusion, a mingled heap; a term used in speaking of the world while yet without form; a Greek word, signifying a confused ma.s.s.

_Alluvial_, deposited from water.

Of what is this last compounded?

The Alluvial formation is composed of sand, gravel, loam, clay, turf, &c., and contains plants, roots, moss, bones, petrified wood, and skeletons of animals. It is distinguished from the Tertiary formation chiefly by its superior position, and by extending over regions where existing streams or other causes now in action could have produced it.

Some geologists mention another formation called the Volcanic, because composed of minerals thrown from the crater of a volcano, such as pumice stones, lava, &c.

_Crater_, the mouth or opening of a volcano.

_Petrified_, hardened into stone.

You mentioned Silicious and Argillaceous Earths: is not, then, the earthy covering of our globe of one common character?

No; by earth is understood a combination of many distinct bodies.

Chemists, by separating earths from each other, and from foreign matters connected with them, have discovered nine or ten primitive earths; all of these, except silex, are compounds of oxygen with metallic bases.